Stabilization of aldehydes



Patented Apr. 21, 1942 STABILIZATION 0F ALDEHYDES Richard 0. Roblin, Jr., Old Greenwich, Conn, as-

signor to American Cyanarnid Company, New York, N. Y., a corporation of Maine No Drawing.

Application November 4, 1937,

Serial No. 172,697

7 Claims.

This invention relates to the stabilization of aldehydes.

A number of aldehydes, for example benzaldehyde, sufier seriously from oxidation on standing, frequently producing colored or other undesirable oxidation products. According to the present invention, it has been found that thiourea and its readily oxidizable derivatives and isomers are excellent oxidation inhibitors or stabilizers for aldehydes. some of its mono-substituted alkyl and aryl derivatives, such as ethyl, allyl and. phenyl thi oureas, are also efiective.

While it is not intended to limit the present invention to any particular theory of action, N, N-polysubstituted thioureas are less effective stabilizers and I believe that it is necessary to have at least one free hydrogen on at least one of the amino groups for the compound to act effectively as a stabilizer. Tetra-substituted thioureas are therefore excluded from the scope of the present invention as they are not sufficiently readily oxidized to exert efiective stabilizing action.

The amount of thiourea or its derivatives to be tion increases the stabilization efiect improves but after concentrations of the order of about 0.1 or 0.2% are reached, further increases do not give additional efiects sufficient to compensate for the increased amount of stabilizer required.

Therefore, it is usually preferable to use amounts of the order of about 0.1-0.2%. The invention, of course, is not limited to this range which merely represents a desirable and practical one. Amounts of the thiourea as described above which produce concentrations of a fraction of a percent are too low to have any material elTect in preventing polymerization of aldehydes which are capable of ready polymerization. In the amount of a fraction of a percent, the thioureas act practically purely as anti-oxidants.

The stabilization appears to be effective generally with most aldehydes, particularly with aromatic aldehydes which are subject to oxidation but do not readily polymerize, although the amount of stabilizer will vary with different products. The invention will be described in detail in conjunction with the stabilization of benzaldehyde but it should be understood that other aldehydes behave similarly, such as vanillin, cinnamic aldehyde, phenylacetaldehyde, acetalde- Thiourea itself is very efiective and hyde, butyraldehyde, lauric aldehyde and the like. The parts in the examples are by Weight except in the case of liquids where they are by the corresponding volume.

Example 1 Benzaldehyde was titrated, the titration being effected by dissolving the benzaldehyde in 20 times its volume of ethyl alcohol and titrating electrometrically with 0.5 N NaOH using a glass electrode. This benzaldehyde showed 0.385 gms. of benzoic acid in the 10 cc. sample on titration. The benzaldehyde was divided into two portions, to one of which was added 0.1% of thiourea. The samples were subjected to the radiation from a 450 Cooper-Hewitt mercury arc lamp at a distance of 12 while a rapid stream of air was passed through both samples for 30 minutes. At the end of this time, the samples were closed in their containers and allowed to stand for 48 hours. The samples were taken and. titrated to determine the amount of benzoic acid obtained. The blank to which no thiourea had been added showed 1.005 gms. of benzoic acid, whereas the sample with thiourea showed only 0.394 gm. This represents only $6 as much oxidation with the thiourea as in the blank.

The action of thiourea which compared with that of hydroquinone which is a standard antioxidant. The sample containing hydroquinone was taken from the same lot of benzaldehyde and was treated in the same manner as the blank and sample containing thiourea. On titration, the hydroquinone sample showed 0.452 gm. of benzoic acid. Thiourea is therefore somewhat over twelve times as effective as hydroquinone. A further important advantage in the use of thiourea lies in the fact that it combats color much more strongly than does an anti-oxidant, such as hydroquinone. Thus the benzaldehyde sample containing thiourea was straw colored whereas the sample containing hyclroquinone was very deeply colored.

Example 2 A test was made as described in Example 1 but instead of using thiourea, allyl thiourea was used. In this case, the blank before treatment showed 0.324 gm. benzoic acid and after treatment 0.857 gm., whereas the sample with allyl thiourea showed only 0.335 gm.

What I claim is:

l. A stabilized aromatic aldehyde containing not more than a fraction of a .percent of a thiourea having at least one free hydrogen attached to one of the thiourea nitrogens.

2. A stabilized aromatic aldehyde containing not more than a fraction of a percent of a thiourea having at least three free hydrogens attached to the thiourea nitrogens.

3. A stabilized aromatic aldehyde containing not more than a fraction of a percent of thiourea.

4. Stabilized benzaldehyde containing not more than a fraction of a percent of a thiourea having at least one free hydrogen attached to a thiourea nitrogen.

5. Stabilized benzaldehyde containing not more than a fraction of a, percent of a thiourea having at least three free hydrogens attached to the thiourea nitrogens.

6. Stabilized benzaldehyde containing not more than a fraction of a. percent of thiourea.

'7. Stabilized benzaldehyde containing not more than a fraction of a percent of allyl thiourea.

RICHARD O. ROBLIN, JR. 

